It's safe to say there will most likely be some degree of cpu bottlenecking even in high resolutions (ie. HL2); just in general. Still, what matters, is how severe it is. For games, as long as you have above P4 3.0ghz or A64 3000+, with these processors or above, getting a faster videocard nets a much greater benefit in games. As long as this condition remains true, then there is little worry about cpu bottlenecking. The article linked shows that A64 1800mhz is sufficient enough to meet such a condition. In other words, when cpu bottlenecking is present, you have to evaluate how much of a performance increase are you happy with if you get a new videocard. For instance, if G70 gives you 80% of its increase as opposed to 100% because of the slower cpu, is it stillw worth it? what about 60/40/20%? You get the idea. A good frame of reference would be to see what resolution and settings you prefer to play at and see how much cpu speed matters at those specific settings.
In your case the 6600GT is the bottleneck in general.